Toyota Prius Ad We'll Never See: Animals Worse Than Hybrids

Some of the first emissions regulations in the world came about because of the way the Los Angeles Basin is shaped.

Reports of smog in southern California started with the Conquistadors, and by the Sixties, LA was notorious for its foul air. California began regulating auto emissions before the U.S. Environmental Protection Agency even existed, and motor vehicles have been a focus of emissions laws ever since.

So-called criteria pollutants (carbon monoxide, hydrocarbons, and nitrous oxides) have been reduced by more than 90 percent since 1970. Vehicle emissions of greenhouse gases, primarily carbon dioxide (CO2), are inversely proportional to gas mileage: The higher the mileage, the lower the CO2 output.

With fuel economy levels set to rise to 34 mpg by 2016, and total U.S. vehicle miles traveled flat or increasing just slightly, many environmental analysts believe greenhouse gas emissions from vehicles may actually decline over the coming decade.

But there's another segment of industry that emits massive amounts of carbon as well, and does so completely and totally unregulated.

It's food production and, more specifically, the highly carbon-intensive business of raising animals for meat. According to the EPA, fully 20 percent of methane emissions in the U.S. come from livestock.